Local polishing method, local polishing device, and corrective polishing apparatus using the local polishing device

ABSTRACT

Provided is a local polishing technique suitable for corrective polishing. Press polishing is performed while supplying a polishing solution between a work and a work-polishing rotating tool locally pressed against the work, the polishing solution having abrasive grains composed of organic particles with an average particle size of 5 μm or more dispersed in a liquid. The rotating tool is made of an elastic material.

TECHNICAL FIELD

The present invention relates to a local polishing method and a localpolishing device, which can be suitably used for corrective polishing.

BACKGROUND ART

In machining of an optical element such as an optical lens, for example,corrective polishing is performed. In the corrective polishing, a work(workpiece) is entirely scanned while a rotating tool, which is capableof local polishing, is being pressed against the work at the same timeof supplying polishing slurry composed of fine abrasive grains with anaverage particle size of approximately 1 μm between the work and therotating tool. In the corrective polishing, a static machining mark isobtained by polishing a material of the same quality as the work inadvance without scanning the rotating tool, and a shape of the staticmachining mark per unit time is obtained. Then, corrective machining toa desired shape is performed by a process of performing deconvolution(deconvolution integral) calculation for this unit machining shape and ashape error (correction target shape) on the work to calculate aresidence time (scanning speed) of the rotating tool and then scanningthe rotating tool along a distribution of the residence time (forexample, refer to Patent Literature 1).

In the corrective polishing, the scanning is performed as mentionedabove. Accordingly, it is important that a local machining amount by therotating tool is linear with time and stable fora long period of time.However, in the conventional corrective polishing, there has been aproblem that it is difficult to obtain a stable machining amount sincethe rotating tool itself that is locally pressed against the work isworn and pressing force is liable to fluctuate. Moreover, since asurface texture of the tool largely affects transportation and retentionof the fine abrasive grains, discharge of chips, and the like, therotating tools are used for the polishing after the surface texture isprepared by pretreatment such as truing (shape formation) and dressing(toothing). However, such pretreatment causes a decrease in machiningefficiency and an increase in cost. In addition, since the rotating toolis easy to wear as described above, it is necessary to frequentlyperform a maintenance operation on the surface shape, which causes afurther decrease in efficiency and a further increase in cost.

On the other hand, in recent years, a non-contact elastic emissionmachining (EEM) method and a polishing method using a magnetic fluid asa tool have also been being used (for example, refer to PatentLiterature 2). In the EEM method, the rotating tool is not activelypressed against the work, but a gap equal to or larger than the particlesize of the fine abrasive grains is maintained between the rotating tooland the work, and the fine abrasive grains in the polishing slurryflowing therebetween and the surface of the work are chemically bondedto each other and are subjected to adhesion removal, whereby precisionpolishing is performed. However, these non-contact machining methodsrequire a circulation device that thoroughly controls viscosity andconcentration of the polishing slurry, and equipment thereof tends tobecome large in size. Moreover, in order to speed up a flow of theslurry passing through the gap, it is necessary to set an outer diameterof an outer peripheral surface of the rotating tool, which faces thework, to a predetermined value or more, and also to set a rotation speedof the rotating tool to a predetermined value or more, and there alsooccurs a certain limit to a correctable spatial resolution.

The correctable spatial resolution in the corrective polishing relatesto the size of the unit machining shape. For example, when undulationswith a period of 1 mm are corrected, it is obvious that the undulationscannot be corrected unless a size of the unit machining shape is 1 mm orless. A non-contact corrective polishing device currently in widespreaduse has a limit of spatial resolution of approximately 1 mm. Further,even in research on the corrective polishing technique, which aims to ahigher resolution, a spatial resolution therein exhibits up toapproximately 0.3 mm. On the other hand, an optical element using ashort wavelength light source such as X-ray is required to have accuracyin the order of single nanometer; however, in an X-ray optical elementseparately developed by the inventors of the present invention, aneffect of wave surface error due to an undulation region with a spatialwavelength of approximately 0.1 to 0.3 mm is confirmed. The non-contactcorrective polishing technique cannot cope with correction of such smallundulations of the spatial wavelength.

CITATION LIST Patent Literature

Patent Literature 1: Japanese Unexamined Patent Application PublicationNo. 2005-22005

Patent Literature 2: Japanese Examined Patent Application PublicationNo. H2-25745

SUMMARY OF INVENTION Technical Problems

In view of the above-mentioned circumstances, what the present inventionseeks to solve is to provide a local polishing technique that issuitable for the corrective polishing, the local polishing techniquepreventing cost from increasing with a simple structure, enabling adevice to be miniaturized, also stabilizing a machining amount by thelocal polishing, and also being capable of obtaining a high spatialresolution.

Solutions to Problems

As a result of diligent examinations to solve the above-mentionedproblems, the inventors of the present invention found the following.That is, the fact that a tool itself is worn and an effect of thesurface texture of the tool increases in the conventional localpolishing using pressing is caused by the fact that a rotating tool 11comes into direct contact with a work 9 since the rotating tool 11 ispressed against a machining target surface 90 of the work 9 whileinterposing fine abrasive grains 81 with an average particle size of 1μm or less therebetween as illustrated in FIG. 1A. Then, the inventorsof the present invention conceived that, if coarser abrasive grains 81with an average particle size of 5 μm or more were used as illustratedin FIG. 1B, then the direct contact between the rotating tool 11 and thework 9 and the wear of the rotating tool 11 due to the direct contactwould be prevented, the surface texture of the tool surface would notaffect the machining, and the machining amount could be stabilized whilepreventing cost from increasing with a simple structure. Then, theinventors of the present invention proceeded with further examinations.

The inventors of the present invention first performed press polishingwith a rotating tool using silica, as polishing abrasive grains, havingan average particle size of 14 μm. As a result, machining stability wasconfirmed. However, there occurred a problem that surface roughnessafter the polishing was greatly deteriorated (see the result ofComparative example 1 of Raster scan machining test 1 to be describedlater).

On the other hand, from the Stokes equation, it is seen that asedimentation rate of particles is proportional to a square of aparticle size. That is, dispersibility also becomes a bottleneck inconsideration of use of polishing abrasive grains with a particle sizeof 10 μm or more, which is set this time. Since a density of usuallyused polishing abrasive grains is 2 to 8 g/cm³, it is difficult toimprove the dispersibility as long as the solution is pure water(density: 1 g/cm³). Actually used aggregated silica had poordispersibility, and it was very difficult to handle the aggregatedsilica in terms of storage and re-stirring though the aggregated silicawas stabilized in the slurry circulation.

The inventors of the present invention considered using, as abrasivegrains, monodispersive particles with an average particle size of 5 μmor more and a low density, apart from the general concept of “polishingabrasive grains” based on metal oxides and metal carbides, and thenconceived to use organic particles. For example, particles of urethane,acrylic, styrene and the like, which are polymer materials, are producedby an emulsion polymerization method. These have a substantiallyspherical shape, and it is possible to produce those having a particlesize of 5 to 10 μm or more. Further, the organic particles are resin.The organic particles are inexpensive, have a low density, have gooddispersibility, and also have good detergency after polishing so as tobe solved in an organic solvent such as acetone. As described above, avariety of advantages are considered in polishing.

Then, as a result of actually performing the press polishing withacrylic particles (average particle size: 10 μm), excellent machiningstability was obtained, and at the same time, machining in which surfaceroughness was maintained was able to be achieved, and the presentinvention was completed.

That is, the present invention includes the following inventions.

(1) A local polishing method including press polishing performed whilesupplying a polishing solution between a work and a work-polishingrotating tool locally pressed against the work, the polishing solutionhaving abrasive grains composed of organic particles with an averageparticle size of 5 μm or more dispersed in a liquid. Here, the averageparticle size refers to a median diameter in a particle sizedistribution measured by the laser diffraction/scattering method.

(2) The local polishing method according to (1), in which the rotatingtool is made of an elastic material.

(3) The local polishing method according to (1) or (2), in which theliquid is pure water or a liquid containing water as a main component.

(4) The local polishing method according to any one of (1) to (3), inwhich the rotating tool includes: a rotating body; a shaft body that hasa tip end provided with the rotating body and is long in an axialdirection around which the rotating body is rotated; and a rotationsupport portion that supports the shaft body on a base end side thereoffor allowing the shaft body to rotate around an axis center thereof, andthe rotating body is pressed, at an outer circumferential surfacethereof, against the work to curve the shaft body, and elastic restoringforce of the curved shaft body causes the rotating body to be pressedand urged against the work.

(5) The local polishing method according to any one of (1) to (4), inwhich an outer diameter of a polishing action region on an outercircumferential surface of a rotating tool, the outer circumferentialsurface facing the work, is set to 5.0 mm or less.

(6) The local polishing method according to any one of (1) to (5), inwhich the organic particles are acrylic particles or urethane particles.

(7) A local polishing device including: a work-polishing rotating toollocally pressed against a work; and machining solution supply means forsupplying, between the rotating tool and the work, a polishing solutionin which abrasive grains composed of organic particles with an averageparticle size of 5 μm or more are dispersed in a liquid.

(8) The local polishing device according to (7), in which the rotatingtool is made of an elastic material.

(9) The local polishing device according to (7) or (8), in which theliquid is pure water or a liquid containing water as a main component.

(10) The local polishing device according to any one of (7) to (9), inwhich the rotating tool includes: a rotating body; a shaft body that hasa tip end provided with the rotating body and is long in an axialdirection around which the rotating body is rotated; and a rotationsupport portion that supports the shaft body on a base end side thereoffor allowing the shaft body to rotate around an axis center thereof, therotating body is pressed, at an outer circumferential surface thereof,against the work to curve the shaft body, and elastic restoring force ofthe curved shaft body causes the rotating body to be pressed and urgedagainst the work.

(11) The local polishing device according to any one of (7) to (10), inwhich an outer diameter of a polishing action region on an outercircumferential surface of a rotating tool, the outer circumferentialsurface facing the work, is 5.0 mm or less.

(12) The local polishing device according to any one of (7) to (11), inwhich the organic particles are acrylic particles or urethane particles.

(13) A corrective polishing device, in which the local polishing deviceaccording to any one of (7) to (12) is used.

Advantageous Effects of Invention

According to the above-described invention of the present application,the wear of the rotating tool is prevented in the local press polishing,the surface texture of the tool surface does not affect the machining,either, and pretreatment and maintenance of the tool surface can beomitted. Then, local polishing, which obtains excellent machiningstability while preventing cost from increasing with such a simplestructure as described above, and can also maintain surface roughness atthe same time, can be achieved, and can be preferably used as thecorrective polishing.

Moreover, when the rotating tool is made of an elastic material, thework is polished by a rolling action of the abrasive grains between therotating tool and the work, thus enabling the further improvement insurface roughness. In addition, the elastic deformation of the rotatingtool stabilizes the pressing force, thus enabling the furtherimprovement in machining stability.

Moreover, when the liquid that allows the organic particles as abrasivegrains to be dispersed therein is pure water or a liquid containingwater as a main component, the dispersibility of the organic particlesis improved, and the machining stability is further improved. Inaddition, when the work is silicon, glass or the like, a soft hydratedfilm made of water is formed on a surface thereof, whereby the removalis further promoted, and the surface roughness can also be improved.

Moreover, the rotating tool is composed of: a rotating body; a shaftbody that has a tip end provided with the rotating body and is long inan axial direction in which the rotating body is rotated; and a rotationsupport portion that supports the shaft body on a base end side thereoffor allowing the shaft body to rotate around an axis center thereof, therotating body is pressed, at an outer circumferential surface thereof,against the work to curve the shaft body, and elastic restoring force ofthe curved shaft body causes the rotating body to be pressed and urgedagainst the work. In this case, the pressing force of the rotating toolis stabilized, thus enabling the further improvement in machiningstability.

When the outer diameter of a polishing action region on an outercircumferential surface of the rotating tool, which faces the work, isset to 5.0 mm or less, local polishing with a higher resolution isenabled.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is an explanatory view for explaining a machining principle.

FIG. 1B is an explanatory view for explaining the machining principle.

FIG. 2 is a front view illustrating a local polishing device accordingto a typical embodiment of the present invention.

FIG. 3 is a perspective view of the local polishing device viewed fromdiagonally below.

FIG. 4 is an explanatory view illustrating a main part of the localpolishing device.

FIG. 5 is a front view illustrating a local polishing device accordingto another embodiment.

FIG. 6 is an explanatory view illustrating a main part of the localpolishing device.

FIG. 7 is an explanatory view illustrating a main part of a rotatingtool.

FIG. 8A is a surface observation image of a static machining markaccording to Example 1.

FIG. 8B is a surface observation image of a static machining markaccording to Example 8.

FIG. 8C is a surface observation image of a static machining markaccording to Example 9.

FIG. 9 is a surface observation image of a static machining markaccording to Comparative example 1.

FIG. 10A is a graph illustrating a relationship between a machiningamount and a machining time according to Example 1.

FIG. 10B is a graph illustrating a relationship between a machiningamount and a machining time according to Comparative example 1.

FIG. 11 is an explanatory view illustrating a method of a raster scanmachining test.

FIG. 12 is a surface observation image of a raster scan machining resultaccording to Comparative example 1.

FIG. 13 is a surface observation image of surface roughness beforeraster scan machining.

FIG. 14A is a surface observation image of surface roughness afterraster scan machining according to Example 1.

FIG. 14B is a surface observation image of surface roughness afterraster scan machining according to Example 2.

FIG. 14C is a surface observation image of surface roughness afterraster scan machining according to Example 8.

FIG. 15A is a surface observation image of surface roughness afterraster scan machining according to Comparative example 1.

FIG. 15B is a surface observation image of surface roughness afterraster scan machining according to Comparative example 2.

FIG. 16A is a graph illustrating a relationship between a machiningamount and pressing force on a synthetic quartz glass substrate ofExample 1.

FIG. 16B is a graph illustrating a relationship between a machiningamount and pressing force on a silicon substrate of Example 1.

FIG. 17 is a surface observation image of a raster scan machining resulton the silicon substrate according to Example 1.

FIG. 18 is a graph illustrating a relationship between a machiningamount and a particle size of abrasive grains.

FIG. 19 is a graph illustrating a relationship between a machiningamount and an abrasive grain concentration.

FIG. 20 is a figure of an ideal machining result (ideal machining amountcalculation result) and a surface observation image illustrating anactual machining result according to Corrective polishing test 1.

FIG. 21 is a comparison diagram between cross sectional profiles(machining amounts) at the respective centers, in the horizontaldirection, of the ideal machining result and the actual machiningresult, according to Corrective polishing test 1.

FIG. 22 is surface observation images of surface roughnesses before andafter Corrective polishing test 1.

FIG. 23 includes a figure of an ideal machining result (ideal machiningamount calculation result), and a surface observation image of an actualmachining result according to Corrective polishing test 2.

FIG. 24 is surface observation images of surface roughnesses before andafter Corrective polishing test 2.

DESCRIPTION OF EMBODIMENTS

Next, embodiments of the present invention will be described in detailwith reference to the accompanying drawings.

As illustrated in FIGS. 2 to 4, a local polishing device 1 according toa typical embodiment of the present invention includes: a work-polishingrotating tool 11 locally pressed against a work 9, and machiningsolution supply means 12 for supplying, between the rotating tool 11 andthe work 9, a polishing solution 8 in which abrasive grains composed oforganic particles with an average particle size of 5 μm or more aredispersed in a liquid.

As illustrated in FIG. 1B, a machining principle is that the abrasivegrains 81 in the polishing solution 8 supplied between the rotating tool11 and the work 9 are sandwiched therebetween and roll on the surface ofthe work 9, so that the surface of the work 9 is polished. Therefore, apolishing action region of an outer circumferential surface of therotating tool 11, which faces the work 9, does not need to adjust asurface texture thereof, and in this example, is composed of an elasticbody made of an elastic material that can capture the abrasive grains 81easily.

The device illustrated in FIGS. 2 to 4 is configured as a type of devicefor polishing such a plate-shaped work 9, and is provided with a workholding mechanism 13 (X-axis stage 41, Y-axis stage 42, Z-axis stage 43)that holds the work 9 with the machining target surface 90 facingdownward so as to be capable of moving the work 9 in XYZ-directions atan upward position. Further, at a position substantially directly belowthe work 9 held by the work holding mechanism 13, provided as themachining solution supply means 12 is a machining solution injectionunit 14 including an injection nozzle 30 that injects the polishingsolution 8 directly above with a gap s1 into which the rotating tool 11is inserted.

In this example, the machining target surface 90 of the plate-shapedwork 9 is flat; however, even if the machining target surface 90 is acurved surface such as a surface of a lens, the curved surface can bedealt with by moving the work 9 three-dimensionally by the work holdingmechanism 13. If the work holding mechanism 13 is provided with therotating mechanism (rotating stage; θ stage) that rotates the work 9while holding the same, then a posture of the work can be changed inaddition to the position thereof, and a degree of freedom in machiningcan be further enhanced.

In this example, a support base 23 that supports the rotating tool 11 inan inclined state is provided with a mechanism that supports a rotationsupport portion 22 so that an angle thereof is adjustable, and can moreflexibly deal with the shape of the machining target surface 90. Thus,the degree of freedom in machining is enhanced. Operations of these workholding mechanism 13 and support base 23 are automatically controlled bya computer (not shown), whereby the local polishing device 1 can be usedas a corrective polishing device that automatically scans the machiningtarget surface.

The machining solution injection unit 14 as the machining solutionsupply means 12 is composed of: the injection nozzle 30; a recovery tank31 around the injection nozzle 30, which receives the polishing solution8 injected from the nozzle, hits a work's machining target surface 90and drops; and a pump 35 that supplies the polishing solution 8, whichis received and recovered into the recovery tank 31, again to theinjection nozzle 30 and injects the polishing solution 8 upward. Duringmachining, the polishing solution 8 circulates between the injectionnozzle 30, the work's machining target surface 90, the recovery tank 31,and the pump 35.

The polishing solution 8 is injected and supplied to the works machiningtarget surface 90 by the machining solution injection unit 14 in thisway, whereby the polishing solution 8 can be stably and efficientlysupplied to the local polishing region of the machining target surface90, and stable polishing for a long time can be achieved with a smallamount of polishing solution. In addition, the local polishing device 1can be used regardless of the shape and size of the work, which alsocontributes to cost reduction. In particular, the polishing solution 8can be supplied evenly in all directions by being injected from directlybelow the machining target surface 90 in the form of a fountain, amachining rate is stabilized more, and an amount of water can also bereduced.

The rotating tool 11 is made of an elastic material. Specifically, therotating tool 11 is composed of: a rotating body 20 made of an elasticmaterial such as rubber; a shaft body 21 that has a tip end providedwith the rotating body 20 and is long in an axial direction in which therotating body 20 is rotated; and a rotation support portion 22 thatrotates the shaft body 21 around an axis center thereof while supportingthe same on a base end side. Then, an outer circumferential surface ofthe rotating body 20 is pressed against the work 9, whereby the shaftbody 21 is curved, and the rotating body 20 is pressed and urged againstthe work 9 by the elastic restoring force of the curved shaft body 21.

As illustrated in FIG. 7, preferably, the rotating body 20 is configuredso that an outer diameter d2 of a polishing action region 201 on anouter circumferential surface 20 a that faces the work 9 (that is, thediameter d2 is a maximum diameter in the region) is 5.0 mm or less. Inthe rotating body 20, a diameter of the rotating body (the diameter is amaximum diameter of the outer circumferential surface thereof) is set tobe smaller than a diameter of the conventional rotating tool. As a shapeof the rotating body 20, various shapes such as a spherical shape, apartial spherical shape, a ring shape having a circular cross section(toroidal shape), and the like can be adopted. Then, the abrasive grainsinterposed between the rotating body 20 and the work's machining targetsurface 90 are rolled on the machining target surface 90 while beingpressed by the rotating body 20 to polish the machining target surface90, and the surface roughness is further improved.

Since the rotating body 20 is made of elastic materials in this way, thepressing force for pressing the machining target surface 90 through theabrasive grains 81 is stabilized, in addition, the abrasive grains 81can be firmly held and rolled on the machining target surface 90, sothat machining stability is improved. As a specific elastic material, itis preferable to use fluorine rubber. Fluorine rubber has a smallfriction of coefficient, and is chemically stable when PH adjustment ofthe machining solution is considered.

Moreover, the rotating body is set to have a small diameter as describedabove, whereby it is considered that force for local pressing will beexerted against the work's machining target surface 90, energy appliedto the intervening abrasive grains for pressing the abrasive grains perunit area will be increased, and the machining rate will also increase.Further, a small unit machining shape is obtained, and an excellentspatial resolution is also obtained.

As the shaft body 21, a shaft of metal such as stainless steel, whichhas a cross section smaller in diameter than the rotating body 20, canbe used. Other materials may be naturally used as long as the materialsare long and flexible in the axial direction. In this example, a tip endportion of the shaft body 21 is fitted and fixed into thetoroidal-shaped rotating body 20, and a base end portion of the shaftbody 21 is fixed to the rotation support portion 22, whereby therotating tool 11 is configured. An electric motor or the like can beused for the rotation support portion 22.

The shaft body 21 is extended diagonally from a position diagonallybelow a gap s1 between the work 9 and the nozzle 30 so that the rotatingbody 20 on the tip end thereof is pressed against the machining targetsurface 90. The base end portion is rotatably supported by the rotationsupport portion 22 provided at the position located below. The axialdirection of the shaft body is defined to be an axial direction at aposition of the tip end provided with the rotating body when the shaftbody is slightly curved by the pressing against the work. In thisexample, the shaft body is provided at an angle of approximately 45degrees with respect to the normal line of the machining target surface90; however, the angle is not limited to that angle. Such a structure inwhich the rotating body 20 is inserted into the gap s1 by the shaft body21 from diagonally below is adopted as in this example, whereby, whenthe machining target surface is a curved surface (for example, a freecurved surface), it is easy to machine the work while rotating the same.When the shaft body 21 is placed in parallel to the machining targetsurface 90, it is difficult to rotate and machine the work in this way.

As in this example, the flexible shaft body 21 is used, and the work 9is machined while the rotating body 20 is pressed and urged against thework 9 by the elastic restoring force of the curved shaft body 21. Inthis way, a positional relationship between the rotating tool 11 and thework 9 may be slightly deviated due to the shape of the machining targetsurface 90. Even in such a situation, the shaft body 21 is onlyelastically deformed by that amount, a large fluctuation in the pressingforce can be avoided, and the pressing force is kept substantiallyconstant, whereby a stable machining amount is obtained. This means thatstrict accuracy is not required for the stages 41, 42 and 43 (workholding mechanism 13) (for example, an accuracy of approximately 10 μmis sufficient even in nano-level corrective polishing of a lens).

The liquid of the polishing solution 8 is preferably pure water or aliquid containing water as a main component in terms of dispersibilityof organic particles. Various organic particles can be adopted, andthose having a density close to 1 g/cm³, such as acrylic particles,urethane particles, and styrene particles, which are made of polymermaterials, are particularly preferable. Among them, urethane and acrylic(both densities are 1.2 g/cm³) are more preferable. Organic particleshave a density closer to 1 g/cm³ than metal oxide particles which aregeneral abrasives, and are easy to disperse without precipitating.Organic particles of different materials may be mixed.

Moreover, an average particle size of the organic particles ispreferably 5 μm or more and 30 μm or less.

Then, referring to FIGS. 5 and 6, a description will be given of anembodiment of another device configuration according to the presentinvention, which is suitable when the surface (rotating surface) of awork having a rotating body shape is used as a machining target surface.Here, the surface of the work includes an outer circumferential surfaceof a cylindrical work such as a rod lens, an outer circumferentialsurface of a conical or truncated cone-shaped work, and an inner/outercircumferential surface of a cylindrical work.

Like the device 1 of the above-mentioned typical embodiment, a localpolishing device 1A according to this embodiment includes: the rotatingtool 11 locally pressed against the work 9; and the machining solutionsupply means 12 for supplying, between the rotating tool 11 and the work9, the polishing solution 8 in which abrasive grains composed of organicparticles with an average particle size of 5 μm or more are dispersed ina liquid. The local polishing device 1A polishes the surface of the work9 according to the same machining principle.

The work holding mechanism 13 is provided with a mechanism that rotatesa columnar or cylindrical work around an axis center thereof togetherwith XYZ stages (not shown). In the machining of the innercircumferential surface/outer circumferential surface of the columnar orcylindrical work, it is not necessary to move the work horizontally to alarge extent. Accordingly, in this embodiment, a machining tank 32having an upper end opening, which houses the polishing solution 8, isprovided as the machining solution supply means 12. In addition, thework 9 held by the work holding mechanism 13 is immersed from abovethrough the opening of the machining tank 32, and in a similar way, therotating tool 11 is immersed from diagonally above through a gap betweenthe opening and the work. Then, the machining target surface 90 as theouter circumferential surface of the work is polished in the liquid bythe rotating tool 11.

In order to stir the polishing solution 8 in the machining tank 32, themachining tank 32 is installed on a magnetic stirrer 33, and a stirrer34 provided on the bottom of the machining tank 32 performs stirring bylow-speed rotation.

Besides, since the configuration of the rotating tool 11, theconfigurations of the polishing solution 8 and the organic particles 81contained therein, and other configurations are the same as those of theabove-mentioned typical embodiment, the same structures are denoted bythe same reference numerals, and a description thereof is omitted.

While the embodiments of the present invention have been describedabove, the present invention is not at all limited to these embodiments,and it is a matter of course that the present invention can beimplemented in various forms without departing from the spirit of thepresent invention.

EXAMPLES

Hereinafter, results of various tests performed using polishing slurryof Examples 1 to 9 and Comparative examples 1 to 4 will be described.

(Polishing Slurry)

As shown in Table 1 below, 13 types of polishing slurry of Examples 1 to9 and Comparative examples 1 to 4 were prepared.

TABLE 1 Average Abrasive grain particle size concentration Material (μm)(vol %) Liquid Example 1 acrylic 10 14.3 pure water Example 2 acrylic 1514.3 Example 3 acrylic 15 16.7 Example 4 actylic 10 16.7 Example 5acrylic 30 16.7 Example 6 acrylic 15 8.3 Example 7 acrylic 15 25 Example8 urethane 15 14.3 Example 9 urethane 15 14.3 Fluorinert FC-43Comparative silica 14 8.5 pure water example 1 Comparative acrylic 314.3 example 2 Comparative acrylic 3 16.7 example 3 Comparative no no 0example 4 abrasive abrasive grains grains

(Static Machining Mark Test 1)

Static machining mark tests in which the scanning of the tool wasstopped were performed, with using the four types of polishing slurry ofExamples 1, 8 and 9 and Comparative example 1 and the local polishingdevice according to the above-mentioned typical embodiment illustratedin FIGS. 2 to 4 and 7, under machining conditions (rotation speed of therotating tool, pressing force thereof, machining time) in Table 2 shownbelow. Note that details of the local polishing device are as follows.

-   -   Rotating body of rotating tool: fluorine rubber with toroidal        shape; diameter (d1): 3 mm    -   Shaft body of rotating tool: stainless steel shaft of φ 1 mm    -   The above rotating tool was installed so that the axial        direction of the shaft body defined 55 degrees with respect to        the normal line of the machining target surface, and the outer        diameter (d2) of the polishing action region was set to 2.2 mm.        Here, though the axial direction was slightly curved due to the        pressing of the rotating tool against the work, the axial        direction is defined to be the axial direction at the position        of the tip end provided with the rotating body.    -   Motor of rotation support portion of rotating tool: a motor        capable of controlling the rotation speed in a range of 50 to        4000 rpm

Moreover, the work was made of a synthetic quartz glass substrate, thework's machining target surface was flat, and the rotating tool waspressed by lowering the stage by a predetermined amount after therotating body came into contact with the machining target surface.

TABLE 2 Rotation Pressing Machining Static speed force time machining(rpm) (N) (min) mark result Example 1 2000 0.012 5 FIG. 8A Example 8 3500.012 5 FIG. 8B Example 9 2000 0.012 30 FIG. 8C Comparative 350 0.012 1FIG. 9 example 1

As seen from measurement results of a scanning white interferometers inFIGS. 8A to 8C and FIG. 9, also in the case of each example (FIGS. 8A,8B and 8C) where the liquid of the polishing slurry was purewater/fluorinert and the organic abrasive grains were acrylic/urethane,the local machining was achieved as in the case (FIG. 9) of thepolishing slurry of Comparative example 1 using general polishingabrasive grains. It was confirmed that an inert perfluoro compound(fluorinert) was also effective in addition to water as a liquid of thegeneral polishing slurry.

(Static Machining Mark Test 2)

A test of confirming a change, depending on the machining time, in themachining amount of the static machining mark was performed, with usingthe two types of polishing slurry of Example 1 and Comparative Example 1and the same local polishing device and the same type of work (syntheticquartz glass substrate with a flat work's machining target surface) asin the above static machining mark test 1, under machining conditions(rotation speed of the rotating tool, pressing force thereof) in Table 3shown below.

TABLE 3 Rotation speed Pressing force Machining (rpm) (N) amount resultExample 1 2000 0.012 FIG. 10A Comparative 600 0.006 FIG. 10B example 1

From graphs of FIGS. 10A and 10B, it is seen that the machining amountis proportional to the machining time in each of Example 1 andComparative example 1. It was confirmed that the machining amount of thestatic machining marks, which was required for the corrective polishing,was proportional to the time.

(Raster Scan Machining Test 1)

An area with 2.5 mm square was raster-scanned, as illustrated in FIG.11, in steps of 10 μm, with using the five types of polishing slurry ofExamples 1, 2 and 8 and Comparative Examples 1 and 2, and the same localpolishing device and the same type of work (synthetic quartz glasssubstrate with a flat work's machining target surface) as in the abovestatic machining mark test 1, under the machining conditions (rotationspeed of the rotating tool, pressing force thereof, machining time) ofTable 4 shown below. As a result, in each of the examples and thecomparative examples, the same raster scan removal as in FIG. 12(Comparative example 1) was confirmed. Moreover, the surface roughnessof each example was evaluated using an RMS value of 0.187 mm×0.14 mmusing a scanning white interferometer.

TABLE 4 Rota- tion Pressing Machining speed force time (rpm) (N) (hour)Result Example 1 2000 0.012 2 RMS 0.169 nm FIG. 14A Example 2 2000 0.0122 RMS 0.225 nm FIG. 14B Example 8 1700 0.012 2 RMS 0.368 nm FIG. 14CComparative 350 0.012 1 RMS 0.488 nm FIG. 15A example 1 Comparative 20000.012 2 RMS 0.557 nm FIG. 15B example 2 Before — — — RMS 0.175 nm FIG.13 machining

In the case of Comparative example 1 (silica particles) and Comparativeexample 2 (acrylic particles with an average particle size smaller than5 μm), the surface roughness has deteriorated considerably after themachining as seen from comparison of FIGS. 15A and 15B with FIG. 13showing a state before the machining. On the other hand, in the case ofExamples 1, 2 and 8 (acrylic particles/urethane particles with anaverage particle size of 10 μm or more), relatively good surfaceroughness is maintained even after the machining as seen from similarcomparison of FIGS. 14A, 14B and 14C with FIG. 13 showing the statebefore the machining.

(Static Machining Mark Test 3)

FIGS. 16A and 16B show results of machining two types of works(synthetic quartz glass substrate and silicon substrate, each of whichhas a flat work's machining target surface) while changing the pressingforce using the polishing slurry of Example 1 and the same localpolishing device as in the above static machining mark test 1.

As seen from the respective graphs of FIGS. 16A and 16B, the machiningamount is proportional to the pressing force.

(Raster Scan Machining Test 2)

An area with 1.0 mm square in the work (silicon substrate with a flatwork's machining target surface) was raster-scanned in steps of 10 μmunder machining conditions of 2000 rpm as the rotation speed of therotating tool, 0.006 N as the pressing force of the rotating tool, and29 minutes as the machining time, with using the polishing slurry ofExample 1 and the same local polishing device as in the above staticmachining mark test 1. FIG. 17 shows measurement results of a scanningwhite interferometer.

From the results shown in FIG. 17, it was demonstrated that it waspossible to also machine a silicon substrate like the machining (FIG.12) for the glass substrate.

(Static Machining Mark Test 4)

The work (synthetic quartz glass substrate with a flat machining targetsurface) was machined, with using: the totally five types of polishingslurry of Examples 3 to 5 and Comparative example 3, which are differentfrom one another only in average particle size, and of Comparativeexample 4 that did never contain abrasive grains but contained only purewater; and the same local polishing device as in the above staticmachining mark 1, under the same machining conditions (1600 rpm as therotation speed of the rotating tool, 0.012 N as the pressing force ofthe rotating tool, and one minute as the machining time). FIG. 18 showsresults of the machining.

As shown in the graph of FIG. 18, the polishing amount increased up to aparticle size of 15 μm; however, the machining amount decreased at theparticle size of 30 μm.

(Static Machining Mark Test 5)

The work (synthetic quartz glass substrate with a flat machining targetsurface) was machined, with using: the four types of polishing slurry ofExamples 3, 6 and 7 and Comparative example 4, which are different fromone another only in abrasive grain concentration; and the same localpolishing device as in the above static machining mark test 1, under thesame machining conditions (1600 rpm as the rotation speed of therotating tool, 0.012 N as the pressing force of the rotating tool, andone minute as the machining time). FIG. 19 shows results of themachining.

As shown in the graph of FIG. 19, as the abrasive grain concentrationincreased, the machining amount also increased. It was found that, asthe concentration was further increased, the increase in the machiningamount slowed down.

(Corrective Polishing Test 1)

It was tested whether a shape error with a period of 0.1 mm in the work(synthetic quartz glass substrate with a flat machining target surface)could be correctively polished, with using the polishing slurry ofExample 1 and the same local polishing device as in the above staticmachining mark test 1.

Any target shape with a width of 0.1 mm was prepared, and deconvolutioncalculation thereof was conducted with a unit machining shape calculatedbased on static machining marks obtained from the static machining marktest 1 of Example 1, whereby a residence time distribution wascalculated. Scanning according to the residence time distribution wasconducted on the synthetic quartz glass flat substrate, and measurementwas performed with a scanning white interferometer. As a result, a shapeextremely close to the ideal was able to be produced (FIGS. 20 and 21).

Moreover, as shown in FIG. 22, the surface roughness region was alsoalmost unchanged and maintained a state before the machining. Thesurface roughness was evaluated using an RMS value of 0.187 mm×0.14 mm.

(Corrective Polishing Test 2)

It was tested whether a shape error with period of 0.15 mm in a work(columnar lens made of φ 10 mm synthetic quartz glass in which an outercircumferential surface was a machining target surface) could becorrectively polished, with using the polishing slurry of Example 1 andthe column or cylinder machining local polishing device according to theabove-mentioned embodiment illustrated in FIGS. 5 and 6.

Like the above corrective polishing test 1, any target shape with awidth of 0.15 mm was prepared, and deconvolution calculation thereof wasconducted with the unit machining shape obtained from the aforementionedstatic machining mark test 1 of Example 1 as in Corrective polishingtest 1, whereby a residence time distribution was calculated. Scanningaccording to the residence time distribution was conducted on such amachining target surface, and measurement was performed with a scanningwhite interferometer. As a result, a shape extremely close to the idealwas able to be produced (FIG. 23). Moreover, as shown in FIG. 24, aresult that the surface roughness region was also almost unchanged wasobtained. The surface roughness was evaluated using an RMS value of0.187 mm×0.14 mm.

In each of the corrective polishing tests, as a result, a spatialresolution of the ideal target shape was able to be obtained, and thesurface roughness region was also able to be maintained. From the aboveresult, the corrective polishing with a desired 0.1 mm periodic shapewas achieved. This method, which combines a rotating tool and relativelylarge-diameter organic particles, can be defined as a correctivepolishing method with high correction spatial resolution and highstability. In particular, this method is considered to be a sufficientlyuseful technique in the development of high-precision optical elements.

REFERENCE SIGNS LIST

-   -   s1 Gap    -   d1 Diameter    -   d2 Outer diameter    -   1 Local polishing device    -   8 Polishing solution    -   9 Work    -   11 Rotating tool    -   12 Machining solution supply means    -   13 Work holding mechanism    -   14 Machining solution injection unit    -   20 Rotating body    -   20 a Outer circumferential surface    -   201 Polishing action region    -   21 Shaft body    -   22 Rotation support portion    -   23 Support base    -   30 Injection nozzle    -   31 Recovery tank    -   32 Machining tank    -   33 Magnetic stirrer    -   34 Stirrer    -   35 Pump    -   41 X-axis stage    -   42 Y-axis stage    -   43 Z-axis stage    -   81 Abrasive grains    -   90 Machining target surface

1: A local polishing method comprising press polishing performed whilesupplying a polishing solution between a work and a work-polishingrotating tool locally pressed against the work, the polishing solutionhaving abrasive grains composed of organic particles with an averageparticle size of 5 μm or more dispersed in a liquid. 2: The localpolishing method according to claim 1, wherein the rotating tool is madeof an elastic material. 3: The local polishing method according to claim1, wherein the liquid is pure water or a liquid containing water as amain component. 4: The local polishing method according to claim 1,wherein the rotating tool comprises: a rotating body; a shaft body thathas a tip end provided with the rotating body and is long in an axialdirection around which the rotating body is rotated; and a rotationsupport portion that supports the shaft body on a base end side thereoffor allowing the shaft body to rotate around an axis center thereof, andthe rotating body is pressed, at an outer circumferential surfacethereof, against the work to curve the shaft body, and elastic restoringforce of the curved shaft body causes the rotating body to be pressedand urged against the work. 5: The local polishing method according toclaim 1, wherein an outer diameter of a polishing action region on anouter circumferential surface of a rotating tool, the outercircumferential surface facing the work, is set to 5.0 mm or less. 6:The local polishing method according to claim 1, wherein the organicparticles are acrylic particles or urethane particles. 7: A localpolishing device comprising: a work-polishing rotating tool locallypressed against a work; and machining solution supply section thatsupplies, between the rotating tool and the work, a polishing solutionin which abrasive grains composed of organic particles with an averageparticle size of 5 μm or more are dispersed in a liquid. 8: The localpolishing device according to claim 7, wherein the rotating tool is madeof an elastic material. 9: The local polishing device according to claim7, wherein the liquid is pure water or a liquid containing water as amain component. 10: The local polishing device according to claim 7,wherein the rotating tool comprises: a rotating body; a shaft body thathas a tip end provided with the rotating body and is long in an axialdirection around which the rotating body is rotated; and a rotationsupport portion that supports the shaft body on a base end side thereoffor allowing the shaft body to rotate around an axis center thereof, andthe rotating body is pressed, at an outer circumferential surfacethereof, against the work to curve the shaft body, and elastic restoringforce of the curved shaft body causes the rotating body to be pressedand urged against the work. 11: The local polishing device according toclaim 7, wherein an outer diameter of a polishing action region on anouter circumferential surface of a rotating tool, the outercircumferential surface facing the work, is 5.0 mm or less. 12: Thelocal polishing device according to claim 7, wherein the organicparticles are acrylic particles or urethane particles. 13: A correctivepolishing device, wherein the local polishing device according to claim7 is used.